The concepts involved in the present invention relate to detection of code sequence signals in multi-user spread-spectrum communications.
Mobile communication is becoming increasingly popular. The recent revolution in digital processing has enabled a rapid migration of mobile wireless services from analog communications to digital communications. For example, cellular service providers have already deployed substantial digital wireless communication infrastructure, much of which utilizes code division, multiple access (CDMA) technology. Increasingly, development efforts are focusing on techniques for high-capacity communication of digital information over wireless links, and much of this broadband wireless development work incorporates spread-spectrum communications similar to those used in CDMA.
Spread-spectrum is a method of modulation, like FM, that spreads a data signal for transmission over a bandwidth, which substantially exceeds the data transfer rate. Direct sequence spread-spectrum involves modulating a data signal onto a pseudo-random chip sequence. The chip sequence is the spreading code sequence, for spreading the data over a broad band of the spectrum. The spread-spectrum signal is transmitted as a radio wave over a communications media to the receiver. The receiver despreads the signal to recover the information data.
The attractive properties of these systems include resistance to multipath fading, soft handoffs between base stations, jam resistance, and frequency reuse. Frequency reuse, translates into potentially significant capacity increases over earlier systems. In addition, in a multipath environment, the use of RAKE receivers enables the harnessing of the total received energy.
Receiving the direct sequence spread spectrum communications requires detection of one or more spreading chip-code sequences embedded in an incoming spread-spectrum signal as well as subsequent synchronization of the receiver to the detected chip-code sequence. Initial detection and phase synchronization of the spreading chip code sequence(s) in the receiver is commonly known as code acquisition. Although simple correlators have been used in the code acquisition for reception of spread-spectrum signals, faster and more efficient techniques for code acquisition rely on matched filters.
A matched filter essentially matches elements or samples of an input signal to elements of a reference chip-code sequence signal, by multiplying a set of N samples of the input signal with the reference signal, then summing the product terms to determine a value of correlation of the set of samples of the input signal to the reference signal. For a single code, if the correlation value exceeds a threshold, then a decision circuit indicates that there is a match. If there are a plurality of possible codes, the receiver typically uses a bank of matched filters for parallel matching to a plurality of reference codes. If the receiver expects to receive only one transmission at a time, the decision processor selects the reference code having the highest correlation to the input samples as the match to the code contained in the received spread-spectrum signal.
In many applications, however two or more transmitters may transmit at the same time using different spreading codes. This is particularly common in a CDMA environment, although similar situations may arise in other types of communication using direct sequence spread spectrum. If two or more stations are transmitting at the same time, particularly if the receiver must receive the transmissions simultaneously, the receiver must search for and acquire multiple codes at the same time from within a broad-spectrum wireless signal. A number of techniques have been developed for reducing interference caused by signals from multiple transmitters and acquiring the spreading code of one or more of the transmitters from the spread-spectrum signals received over the air-link.
For example, U.S. Pat. No. 5,644,592 proposes a method of decoding a spread spectrum composite signal, involving despreading each coded signal and averaging each despread signal, which is analyzed to produce a tentative decision. The Divsalar method respreads and sums signals to produce combined interference signals, which are scaled and combined with a weighting factor, to produce a scaled combined interference signal. The method also entails scaling the composite signal with the weighting factor to produce a scaled composite signal and scaling the signal value by the complement of the weighting factor to produce a leakage signal. The scaled composite signal, the scaled combined interference signal and the leakage signal are combined to produce an estimate of a respective user signal.
U.S. Pat. No. 5,719,852 discloses a spread-spectrum interference canceler for reducing interference in a direct sequence CDMA receiver having N chip-code channels. The canceler includes a bank of correlators or matched filters, spread-spectrum-processing circuits, subtracting circuits, and channel correlators or channel-matched filters. Using a plurality of chip-code signals, the correlators despread the spread-spectrum CDMA signal. The spread-spectrum-processing circuits use a timed version of the chip-code signals, for spread-spectrum processing each of the despread signals with a respective chip-code-signal corresponding to a respective despread signal. For recovering a code channel using an ith chip-code-signal, the subtracting circuits subtract from the spread-spectrum CDMA signal, each of the Nxe2x88x921 spread-spectrum-processed-despread signals thereby generating a subtracted signal. The Nxe2x88x921 spread-spectrum-processed-despread signals do not include the spread-spectrum-processed-despread signal of the ith channel of the spread-spectrum CDMA signal. The channel correlator or channel-matched filter despreads the subtracted signal.
U.S. Pat. No. 5,757,791 discloses a multistage low-complexity linear receiver for DS-CDMA spread-spectrum communications. Each stage of the multistage linear receiver recreates the overall modulation, noiseless channel, and demodulation process. The outputs of these stages are then linearly combined. The combining weights may be chosen to implement different linear detectors, such as decorrelating and minimum means squared error (MMSE) detectors. Additional stages may be added to the multistage linear receiver to improve performance.
There are several disadvantages to the prior art code techniques. The prior techniques generally rely on multiple stages of correlation or matched filtering, and every stage needs signal regenerators. This requires a lot of hardware. Due to such hardware costs, implementation typically must be limited to three stages or iterations of the algorithm, which typically is not enough for a satisfactory interference cancellation. A further problem is that the processing delay to complete the search algorithm requires at least as many symbol periods as the number of stages or iterations. Also, the prior algorithms do not necessarily produce a definite result.
The approach disclosed in the U.S. Pat. No. 5,644,592 patent, for example requires a lot of hardware and does not guarantee that the algorithm will produce a definitive solution. The approach from the U.S. Pat. No. 5,719,852 patent also does not guarantee that the algorithm will identify an optimal solution for the search for the code or codes actually present in the received signal. The technique of U.S. Pat. No. 5,757,791 is a particularly complex approach and again, it does not produce a definitive solution to the search for the code or codes contained in the spread-spectrum signal.
Hence a need still exists for a more efficient and effective technique for analyzing correlation of a received signals to a plurality of possible codes that may be received from multiple users, to definitely identify the actual code set received at any given time.
Hence a general objective of the invention is to achieve a more effective technique for demodulating a potential plurality of codes in a received spread-spectrum signal to recover simultaneously transmitted data.
Another objective relates to performing the code recognition in a fashion that is much less hardware intensive than the prior art and requires less delay.
The inventive concepts alleviate the above noted problems in spread-spectrum communications and achieve the stated objectives by using a particular type of mathematical algorithm, specifically a Quasi-Newtonian optimization routine, to definitively solve a quadratic matrix equation and determine the one or more of the potential spreading codes actually received in the spread-spectrum signal. Although the process may require multiple iterations of the code search algorithm, all iterations flow from one set of actual signal correlations, i.e., for one symbol interval, which means that all iterations can be completed during a period no longer than a single symbol.
Hence a first aspect of the invention relates to a demodulator for a spread-spectrum receiver system. The demodulator simultaneously demodulates data from a plurality of possible spreading code sequences from a received spread-spectrum signal. The demodulator comprises a bank of comparators and a processor. The comparators, typically correlators or matched filters, receive and process the spread-spectrum signal. Each comparator determines a value representing a level of correlation of the received spread-spectrum signal to a respective one of the possible spreading code sequences. The processor executes an iterative process for solving a quadratic formulation based on the correlation values and the possible spreading code sequences for convergence on a Hessian function. This process identifies a set of the possible spreading code sequences contained within the received spread-spectrum signal, to recover user data. The preferred embodiment of this process is a Davidson-Fletcher-Powell (DFP) algorithm.
The processor may be a programmed processor, such as a digital signal processor (DSP) or possibly a microprocessor and/or math co-processor. Alternatively, the processor may be implemented using discrete components arranged to perform the necessary iterative computations.
Another aspect of the invention relates to a method for simultaneously demodulating data from a plurality of possible spreading code sequences from a received spread-spectrum signal. The method involves receiving a plurality of respective correlation values. Each received value represents the correlation of the received spread-spectrum signal to a respective one of the possible spreading code sequences. The inventive method also involves executing a plurality of iterations of a routine, preferably a Davidson-Fletcher-Powell (DFP) algorithm, for solving a quadratic matrix formulation based on the correlation values and the plurality of possible spreading code sequences for convergence on a Hessian function. The solution, found after a predetermined iteration of the execution of the routine, identifies a set of the possible spreading code sequences contained within the received spread-spectrum signal and thus user data carried by the identified codes.
The present inventions find particular application to recognition of multiple code sequence transmissions from multiple users, e.g. in a CDMA environment. However, the invention also is applicable to spread-spectrum communications, for example, for reception of a signal containing multiple spreading codes carrying multiplexed data from a single source.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.